In the manufacture of photographic emulsions, silver nitrate is typically reacted with halide salts in the presence of gelatin to form a photographically active silver halide. Typical photographic manufacturing processes include the preparation of raw materials in separate kettles (e.g., one kettle for silver nitrate, one kettle for halide salts, one kettle for gelatin, and optional kettles for chemical addenda). After the raw materials are prepared, the silver nitrate and halide salts are transferred at high shear rates into a reaction vessel, usually referred to as the make kettle, which contains water and may contain some gelatin, salts and ripeners. The silver nitrate and halide salts are reacted in the make kettle to form a silver halide suspension, hereafter referred to as an emulsion, containing silver halide grains of a certain size and morphology. This emulsion is typically transferred through a filter after which the emulsion is washed and concentrated. After washing and concentrating the emulsion, the emulsion is transferred into a finish kettle. The concentration of silver in the finish kettle is then determined from which is calculated the total amount of silver in the finish kettle. In many cases, chemical addenda are added to the emulsion in the finish kettle based on the total amount of silver in the finish kettle. These chemical addenda include sensitizing dyes that make an emulsion sensitive to certain wavelengths of light, gelatin to increase the viscosity and achieve certain coating parameters and salts, acids and bases to minimize changes after storage in the final silver halide grain morphology.
In order to manufacture a high quality photographic emulsion as determined by a consistent sensitometric response, the ratio of addenda to silver must be kept as consistent as possible. The more consistent this ratio, the more reproducible will be the amount of addenda adsorbed onto each silver halide grain and the more consistent the sensitometric response of the emulsion. Therefore, sensitometric consistency can be improved by having consistent amounts of silver in the finish kettle.
A second important manufacturing parameter for a photographic emulsion is the quantity of silver as silver halide in the finish kettle. Yields of silver in the finish kettle that approach 100% of the amount of silver prepared in the silver nitrate kettle mean less waste and lower manufacturing costs.
Therefore, both the silver yield and silver yield variability in the finish kettle are important process control parameters that ensure consistent, high quality photographic emulsions manufactured with the lowest possible manufacturing costs. Unfortunately, the silver yield and the silver yield variability in the finish kettle are a result of the silver loss caused by all of the above-described processing steps. If the silver yield in the finish kettle is too low or if the silver loss variability in the finish kettle is too high, it is usually not obvious what part or parts of the emulsion manufacturing process should be improved to reduce the variability and yield problems. It is therefore desirable that the main sources of low silver yield and/or high silver loss variability be identified.
Current methods for determining where silver is lost and where silver loss variability is introduced into an emulsion manufacturing process are to measure the silver yield in each part of the process. For example, once all of the silver nitrate for a batch has been added to the silver nitrate kettle, the concentration of silver times its kettle volume is a measure of the amount of starting silver in the silver nitrate kettle. Once all of the silver is transferred out of the silver nitrate kettle into the make kettle, measurements of the volume and silver concentration of the emulsion in the make kettle can be used to determine how much silver is in the make kettle. The difference between these two measurements is an indication of the silver left in the silver nitrate kettle and the silver nitrate transfer line.
Other measurements that might be used to estimate total silver are the mass of the emulsion based on kettle weigh scales or based on a mass flow meter in a transfer line and a measured density to infer silver concentration. Whatever method is used to determine total silver yield, the problem with these methods is that yields are typically so close to 100% that measuring the silver yields to the needed accuracy of typically less than 0.01% is not practical. These methods of measuring silver yield from each part of the emulsion manufacturing process also cannot be used to differentiate which part of the process is the main cause of the problem.
The present invention is an easily implemented method for determining where silver is being lost in the emulsion manufacturing process and for determining sources of silver loss variability. Once sources of silver loss and/or silver loss variability have been determined, steps can be taken to reduce the problems and thereby increase yields and/or increase the consistency of silver in the finish kettle from one batch to another.